In weakly metamorphosed massive sulfide deposits of the Urals (Dergamysh, Yubileynoe, Yaman-Kasy, Molodezhnoe, Valentorskoe, Aleksandrinskoe, Saf’yanovskoe), banded sulfides (ore diagenites) are recognized as the products of seafloor supergene alteration (halmyrolysis) of fine-clastic sulfide sediments and further diagenesis leading to the formation of authigenic mineralization. The ore diagenites are subdivided into pyrrhotite-, chalcopyrite-, bornite-, sphalerite-, barite- and hematite-rich types. The relative contents of sphalerite-, bornite- and barite-rich facies increases in the progression from ultramafic (=Atlantic) to bimodal mafic (=Uralian) and bimodal felsic (=Baymak and Rudny Altay) types of massive sulfide deposits. The ore diagenites have lost primary features within the ore clasts and dominantly exhibit replacement and neo-formed nodular microtextures. The evolution of the mineralogy is dependent on the original primary composition, sizes and proportions of the hydrothermal ore clasts mixed with lithic serpentinite and hyaloclastic volcanic fragments together with carbonaceous and calcareous fragments. Each type of ore diagenite is characterized by specific rare mineral assemblages: Cu–Co–Ni sulfides are common in pyrrhotite-rich diagenites; tellurides and selenides in chalcopyrite-rich diagenites; minerals of the germanite group and Cu–Ag and Cu–Sn sulfides in bornite-rich diagenites; abundant galena and sulfosalts in barite- and sphalerite-rich diagenites and diverse tellurides characterize hematite-rich diagenites. Native gold in variable amounts is typical of all types of diagenites.

The aim of this paper is the investigation of the role of diagenesis in the transformation of clastic sulfide sediments such as sulfide breccias from the Semenov-3 hydrothermal field (Mid-Atlantic Ridge). The breccias are composed of marcasite–pyrite clasts enclosed in a barite–sulfide–quartz matrix. Primary hydrothermal sulfides occur as colloform, fine-crystalline, porous and radial marcasite–pyrite clasts with inclusions or individual clasts of chalcopyrite, sphalerite, pyrrhotite, bornite, barite and rock-forming minerals. Diagenetic processes are responsible for the formation of more diverse authigenic mineralization including framboidal, ovoidal and nodular pyrite, coarse-crystalline pyrite and marcasite, anhedral and reniform chalcopyrite, inclusions of HgS phase and pyrrhotite–sphalerite–chalcopyrite aggregates in coarse-crystalline pyrite, zoned bornite–chalcopyrite grains, specular and globular hematite, tabular barite and quartz. The early diagenetic ovoid pyrite is enriched in most trace elements in contrast to late diagenetic varieties. Authigenic lower-temperature chalcopyrite is depleted in trace elements relative to high-temperature hydrothermal ones. Trace elements have different modes of occurrence: Se is hosted in pyrite and chalcopyrite; Tl is related to sphalerite and galena nanoinclusions; Au is associated with galena; As in pyrite is lattice-bound, whereas in chalcopyrite it is related to tetrahedrite–tennantite nanoinclusions; Cd in pyrite is hosted in sphalerite inclusions; Cd in chalcopyrite forms its own mineral; Co and Ni are hosted in chalcopyrite.

The trace element (TS) composition of isocubanite, chalcopyrite, pyrite, bornite, and covellite from oxidized Cu-rich massive sulfides of the Ashadze-2 hydrothermal field (12°58′ N, Mid-Atlantic Ridge) is studied using LA-ICP-MS. The understanding of TE behavior, which depends on the formation conditions and the mode of TE occurrence, in sulfides is important, since they are potential sources for byproduct TEs. Isocubanite has the highest Co contents). Chalcopyrite concentrates most Au. Bornite has the highest amounts of Se, Sn, and Te. Crystalline pyrite is a main carrier of Mn. Covellite after isocubanite is a host to the highest Sr, Ag, and Bi contents. Covellite after pyrite accumulates V, Ga and In. The isocubanite+chalcopyrite aggregates in altered gabrro contain the highest amounts of Ni, Zn, As, Mo, Cd, Sb (166 ppm), Tl, and Pb. The trace element geochemistry of sulfides is mainly controlled by local formation conditions. Submarine oxidation results in the formation of covellite and its enrichment in most trace elements relative to primary sulfides. This is a result of incorporation of seawater-derived elements and seawater-affected dissolution of accessory minerals (native gold, galena and clausthalite).

As a result of LA ICP MS analysis of sulfides of the Semenov-2 hydrothermal field, it is established that covellite, which replaces Zn sulfides, is enriched in most trace elements. The Ga, Ni, and In contents in it do not vary, whereas Mn, Co, and Cd are lower than in sphalerite. The distribution of trace elements in covellite, which replaces Cu–Fe sulfides, is distinct: it is enriched in Cd, Sb, Pb, and Bi, whereas the contents of other elements are either lower or invariant. Covellite, which replaces Zn sulfides, is enriched in all trace elements relative to that replacing Cu–Fe sulfides. Enrichment of covellite in trace elements relative to primary sulfides was favored by oxidation of the hydrothermal fluid by seawater, which is similar to the processes of submarine oxidation of ancient massive sulfide deposits. Covellite is also a host to invisible gold and silver in ores of the Semenov-2 field along with toxic elements such as As, Se, Te, Tl, and Cd.

Halmyrolysis, as one of the global processes of alteration of seafloor hydrothermal sediments, needs to be recognized in terms of mineral and trace element evolution to elaborate new criteria for metallogenic and geoecological forecasts with respect to ocean exploration. The purpose of this paper is to explain trace elements’ behavior during the halmyrolysis of sulfide deposits. This task is resolved using an LA-ICP-MS analysis of iron oxyhydroxides (IOHs) on examples of oxidized pyrrhotite-rich diffusers of the ultramafic-hosted Pobeda-1 hydrothermal field (Mid-Atlantic Ridge). The IOHs formed after the sulfides were enriched in seawater-derived trace elements (Na, K, Mg, Ca, Sr, P, U, Mo, V, REE, Cr). Six trace element assemblages (TEAs) are statistically recognized for the IOHs. TEA-I (Cu, In, Sn, Bi, Se, Te) is inherited from chalcopyrite, isocubanine and bornite microinclusions. TEA-II is typical of Zn sulfides (Zn, Cd, Sb, Tl, Ag) interacted with seawater (Mg, U, Mo, Ni, Na, K) and hydrothermal fluid (Eu). TEA-III (Ca, Sr, Cu, Si, Se, P, As) reflects the inclusions of aragonite, opal, atacamite and possibly native selenium, while P and As occur as absorbed oxyanion groups on IOHs or Ca–Fe hydroxyphosphates. TEA-IV (Al, Ga, Ge, Tl, W, Ti ± Mn, Co, Ba) indicates the presence of minor clays, Co-rich Mn oxyhydroxides and barite. TEA-V with Pb and V is closely related to TEA-VI with REEs except for Eu. The halmyrolysis of sulfides includes two stages: (i) oxidation of S(II) of primary sulfides and the formation of supergene sulfides, which scavenge the redox-sensitive elements (e.g., U, Mo, Ni, Eu), and (ii) oxidation of Fe (II) to Fe (III) and absorption of most elements of TEAs III, IV, V and VI by IOHs.